Moment Frames In Combinations with Shear Walls. Large Brace forces.

[tmoe]

I live in SF CA and I'm currently involved in a number of seismic retrofit jobs. These jobs are all wood framed residential units 3 to 4 stories over mostly open garage spaces. Basically your classic soft story type structure. Almost all of them are rectangular in shape aprox 25'X75'
Often these lower levels (in the short direction) have one or two available interior wall lines that can be strengthened to act as a shear wall, a full wall at the rear that is easily strengthened, and an open garage front that begs for a frame.

Per ASCE7 12.2.3.3, if I use a combination of moment frame and shear walls in the short direction, I must use the lowest R value of the two systems. This means that if I use OMF at the front, the forces to my shear walls get factored up by (6.5/3.5)=1.86. This (more often than not) seems to either render the shear wall ineffective (such as the rear wall where we only want to add ply to the inside face to avoid exterior work) or the diaphragm becomes over loaded (straight board sheathing) without adding more wall lines.

Using a SMF would circumvent this issue because I can use the lower R=6.5 to design both the frame and the shear walls, but I've run through a couple designs and the out of plain bracing requirements for the SMF seem impossible to reasonably develop into the wood diaphragm. This seems to be a selling point for proprietary products such as the Simpson Strong Frame SMF which requires no additional bracing Link

Parking is a premium in these units, so adding walls is not really an option. Are ppl out on here using SMF in wood retrofits and if so how are you getting these brace forces to work out? Any tips on getting a OMF in there without the shear walls loads blowing up??

thanks so much.

 

[RM87]

Have you considered placing additional plywood sheathing on the lower face of the joists to increase the diaphragms allowable loads? Keep in mind that the additional stiffness may lend towards diaphragm rigidity.

 

[JAE]

What about a dual system (i.e. the exclusion found in 12.2.3.3)?

Would that get you there?

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[tmoe]

RMassoudi, thanks for the reply. That seems like a potential solution, I will have to give some thought to how to account for different types of sheathing above and below the joists (existing board sheathing above, new ply below). SF is pretty strict on what you can use for allowable values (if any) when combining materials.

JAE, thank you. None of these buildings are 2 stories or less (exception 2) so that wouldn't apply unfortunately.

 

[DaveAtkins]

For the long direction, can you eliminate the OMF along the front and use just the shear wall along the back? Or does IBC not allow a three sided diaphragm in the San Francisco area?

DaveAtkins

 

[jdgengineer]

I'm assuming you are doing this as part of the San Francisco Soft Story Ordinance. If you are, check out AB-107 which provides specific guidance for SF interpretation and modification of building code requirements for these retrofits. You will want to be completing the retrofit to either the IEBC A4 or preferably FEMA P807. FEMA P807 will be more forgiving with combining different materials and R factors as it is creates a non-linear static pushover analysis.

Check out section A6.3 of AB-107. This might sound crazy, but the section modifies the R factor for a cantilevered column system if you meet the requirements. Therefore, per this section you can use an R = 8 for cantilevered columns.

 

[tmoe]

jdgengineer, that is a great reference, thanks for the info.
We use A$ in this office as we typically don't know (or don't get) 2nd floor wall layouts required for the FEMA analysis.

What the the deal with the wording in section A6.3 regarding a cantilevered column system meeting the detailing requirements of special, intermediate, or ordinary. Is this just referring to what column sections are permitted to be used based on 341-10?

 

[jdgengineer]

It's my understanding that A6.3 allows you to use the R factor corresponding to a special moment frame for a cantilevered column provided you meet the requirements in the section. This would allow you to use an R = 8 and Cd = 5.5, instead of R=2.5 and Cd = 2.5 as shown in ASCE 7-10. I would interpret this to mean if you can meet the detailing requirements for Special Cantilevered Column (including bracing requirements) that you can use R for special moment frame provided you meet the rest of the requirements of A6.3.

A6.3. Use of Cantilevered Column Systems. Cantilevered column systems conforming to the following provisions may be considered as moment frame systems (Special, Intermediate, or Ordinary, as applicable, based on detailing) with regard to determination of the R, Ω0, and Cd factors.
1. Columns shall not carry gravity load.
2. Columns shall be configured in pairs (or larger groups) connected by a continuous foundation or grade beam.
3. The continuous foundation or grade beam shall be designed to resist the expected plastic moment at the base of each column, computed as RyFyZ, as defined in AISC 341-10.
4. The flexibility of the continuous foundation or grade beam, considering cracked section properties of reinforced concrete, shall be included in computing the deformation of the cantilevered column system.
5. Cantilevered columns shall be considered as twice their actual height when checking lateral torsional buckling.

 

[tmoe]

Yes, Im in agree with your interpretation, but where exactly are the detailing requirements for a Special or Ordinary Cantilevered Column system??

ASCE7 Table 12.2-1 kicks me to Section 14.1 which then kicks me to AISC 341, but I cant seem to find anything there. Am I overlooking it or in the wrong place?!

 

[jdgengineer]

Chapter 14 of ASCE 7-10 is not adopted by the IBC so that Chapter is not typically applicable. If you are using the current AISC 341-10 there is a section for Cantilevered Columns. I don't have the book in front of me but I believe it is in Chapter E.

 

[tmoe]

Yes, I was overlooking it! Thanks!